Lesson 05: Sedimentary Rocks


 I decided to do my blog post on two different sedimentary rocks. This week has been slightly crazy, so I just snapped some pictures of some samples in the geology lab on campus, so I already knew they were sedimentary because of their labels. This first rock is rock gypsum. Rock gypsum is a chemical sedimentary rock. It is not very hard, being made of gypsum, with a hardness of 2. I was able to scratch this with my fingernail. It is nonclastic (has no clasts) and this sample didn't have any outstanding structures such as layers or ripples. Rock gypsum is formed in evaporating basins or playa lakes. It forms when gypsum precipitates from concentrated solutions and can be used in plaster.


The second sedimentary rock I took pictures of was a sample of shale. Shale is formed when clay deposits get compacted. This rock has clay-sized clasts, but they are so tiny you cannot see them without a really good magnifier. (We couldn't see the clasts in lab.) The clasts are well sorted and rounded though because they have gone through a lot of weathering and wear to get to that tiny size.You can kind of see some layers in this sample. Shale is found in deep marine environments or quiet lakes where the clay clasts can settle and get compacted. Something fun about shale is that it can look a lot like siltstone. The way to tell them apart is by checking their texture...with your mouth. Siltstone feels grainy in your mouth, while shale feels smooth. Overall, sedimentary rocks are very interesting to look at and often have some cool history.

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